1. Field of the Invention
The present invention generally relates to a method and apparatus for underwater pelletizing and subsequent drying of polyethylene terephthalate (PET) polymers. More specifically, the present invention relates to a method and apparatus for underwater pelletizing PET polymers and subsequent drying of the PET polymer pellets in a manner to self-initiate the crystallization process of the PET particles and produce pellets having a desired level of crystalline structure rather than an amorphous structure.
2. Description of the Prior Art
Underwater pelletizing systems for producing pellets of polymeric or other plastic materials has been known for many years. The starting materials such as plastic polymers, coloring agents, additives, fillers and reinforcing agents, and modifiers, are mixed in kneaders. In the process, a melt is produced which is extruded or pressed through dies to form strands which are immediately cut by rotating cutter blades in the water box of the underwater pelletizer. Water with or without additives is continuously flowing through the water box to cool and solidify the polymer strands and pellets and carry the pellets out of the water box through transport piping to a dryer, such as a centrifugal dryer, where the water is removed from the pellets.
For quite some time, the polymer industry has sought to process PET polymers into a pellet shape using underwater pelletizer systems. A major drawback of using underwater pelletizing, as well as other pelletizing systems, for processing PET into pellet shapes is the typically amorphous condition of these pellets when they leave the dryer of the underwater pelletizing system. The amorphous nature of the resulting pellet is caused by the fast cooling of the PET material once introduced into the water flow in the water box of underwater pelletizer and while the water and pellet slurry is being transported by appropriate piping to the dryer.
Typically, increasing the water flow through the water box of the underwater pelletizer and increasing the water temperature, along with pipe dimensional changes and reducing the distance between the pelletizer and dryer unit, does not help to sufficiently maintain the pellet temperature. Under such circumstances, the PET pellets still leave the dryer at a temperature, usually below 100° C., which is below the temperature at which crystallization can occur.
End users of PET polymer pellets typically require that the pellets be in a crystalline state, rather than an amorphous state, principally for two reasons, both relating to the fact that the end user wants to process the PET pellets in a substantially dry condition, with zero or near zero water content. First, PET polymers are very hygroscopic, and crystalline PET pellets absorb considerably less moisture during shipment and storage than amorphous PET pellets. Accordingly, crystalline PET pellets can be dried to the requisite zero or near zero moisture content more easily by the end user. Second, the temperature required to completely dry PET polymers is higher than the temperature at which amorphous PET pellets convert to the crystalline form. Therefore, when drying amorphous PET pellets, it is necessary to first achieve crystallization at the requisite lower temperature before raising the temperature to the drying temperature. Otherwise, the amorphous PET polymer pellets may agglomerate and destroy the pellet form.
As a result, manufacturers of PET pellets must typically subject the amorphous PET pellets to a secondary heating step of several hours at very high temperatures, usually in excess of 80 to 100° C., to change the amorphous structure of the pellets to a crystalline structure. This is a very expensive second step in order to convert the PET polymer pellets into the desired crystalline state.
However, it is recognized by the end users and manufacturers of PET pellets that total (100%) crystallinity of the PET pellets is not necessarily required in order to dry the PET pellets for further processing or use in the Solid State Process (SSP). Rather, a total crystallinity, or crystallinity grade using the Calcium Nitrate measurement method, above 30%, and preferably above 40%, is acceptable for the PET end users.
An alternative approach is disclosed in WO 2004/033174 in which the polymer is granulated or pelletized in a water bath at a temperature of more than 100° C. The resulting pellets may be further treated in the water bath for a defined period of time thereafter, while retaining the high temperature, in order to convert the amorphous material into a crystalline material. This system requires pressurization to maintain the water at the super-boiling point temperature, followed by a pressure reduction procedure.
It is also known generally that air can be injected into the exit stream of a water and pellet slurry from a pelletizer in order to enhance the transport of the water/pellet slurry. See, for example, U.S. Pat. No. 3,988,085.